This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from two applications, WAVELENGTH STABILIZING CIRCUIT PROVIDED WITH A CIRCUIT FOR MONITORING THE STATE OF THE WAVELENGTH OF A LIGHT SIGNAL IN AN OPTICAL COMMUNICATIONS SYSTEM EMPLOYING WAVELENGTH DIVISION MULTIPLEXING filed with the Korean Industrial Property Office on Jul. 30, 1998 and there duly assigned Ser. No. 30893/1998, and WAVELENGTH STABILIZING CIRCUIT PROVIDED WITH A CIRCUIT FOR MONITORING THE STATE OF THE WAVELENGTH OF A LIGHT SIGNAL IN AN OPTICAL COMMUNICATIONS SYSTEM EMPLOYING WAVELENGTH DIVISION MULTIPLEXING filed with the Korean Industrial Property Office on Jun. 28, 1999 and there duly assigned Ser. No. 24668/1999.
1. Field of the Invention
The present invention relates to an optical communications system employing wavelength division multiplexing (WDM), and more particularly a circuit for monitoring the stability of the wavelengths of the light signals generated by the channel transmitters.
2. Description of the Related Art
In an optical communications system, WDM is the sharing of a single optical fiber to transmit a plurality of light signals having different wavelengths. On the contrary, the light signals multiplexed on a single optical fiber are separated by a wavelength division demultiplexing method. The optical communications system employing WDM comprises a plurality of channel transmitters, each of which employing a laser diode having a narrow spectrum as the light source. The wavelengths of the channels must be arranged closely with a narrow interval on the order of 100 or 200 GHz between adjacent channels, so that the wavelength of the light signal generated by each laser diode may not exceed the range prescribed for the respective channel. Hence, it is necessary to monitor and control the wavelength of the light signal of each channel so as to be within the prescribed range.
One of the methods for stabilizing the wavelength of the light signal of each channel within the prescribed range is to employ an optical fiber Fabry-Perot filter, which stabilizes a plurality of channels as a whole so that adjacent channels completely separated. An example of this is disclosed in an article entitled xe2x80x9c4-Channel Light Frequency Multiplexingxe2x80x9d authored by Kap Yeol Ryu et al., Journal of Korean Institute of Electronic Engineers, Vol 32, Part A, Section No. 8, pp. 133-138, Article No. 95-32A-8-16.
A wavelength stabilizing circuit provided in an optical communication system employing WDM includes a Fabry-Perot filter as disclosed in the previously cited Ryu et al. article. Such a WDM communication system includes a plurality of transmitters each comprising a laser diode, a laser diode driver, an oscillator, a PSD (phase sensitive detector),an LPF (low pass filter) and an adder. The laser diode driver drives the laser diode to generate a light signal at a frequency prescribed for the particular channel. All of the channel transmitters generate respective light signals having different wavelengths.
All of the light signals generated by the channel transmitters are inputted into the WDM device which multiplexes the light signals. The multiplexed light signals are transmitted through an optical transmission path. The Fabry-Perot filter, photodetector, oscillator, PSD, LPF, and adder serve to stabilize the wavelengths. The frequency of the light signal varies in the range of a few KHz around the resonant frequency of the Fabry-Perot filter and is detected by the phase sensitive detector to stabilize the wavelength. The Fabry-Perot filter and photodetector are commonly used for all of the channels while the oscillator, PSD, LPF, and adder are provided for every channel transmitter.
The Fabry-Perot filter has resonant frequencies continuously arranged with given free spectral ranges according to the central frequencies specified for the channels. The Fabry-Perot filter is connected to the output of the WDM device. The light signal transmitted from the WDM device through a light transmission path is partially branched to the Fabry-Perot filter. The photodetector converts the light signals from the Fabry-Perot filter into respective electrical signals supplied to the PSD""s of the channel transmitters. The oscillator of each channel transmitter generates a particular dither signal supplied to the PSD and adder. The dither signal is a sine wave used to modify the specified central frequency of each channel transmitter so as to vary the frequency of the light signal around the resonant frequency to obtain an error signal for stabilizing the wavelength. The PSD effects a phase sensitive detection of the dither signal and the output signal of the photodetector so as to obtain an error signal corresponding to the first derivative signal of an Airy function which represents the transmission characteristic curve of the Fabry-Perot filter. The LPF filters the error signal to generate a direct current voltage of a level that represents the deviation of the wavelength of the light signal from the specified central frequency. The detected voltage is supplied to the adder and combined with the dither signal from the oscillator to form a bias signal of the laser diode driver and accordingly, the wavelength of the light signal of the laser diode is stabilized within a desired range.
The detected voltage of the LPF may be divided into three parts according to the variation of the wavelength of the light signal of the laser diode. Firstly, if the wavelength precisely agrees with the specified central frequency, the detected voltage is 0V. Secondly, if it slightly deviates from the specified central frequency, the detected voltage has a level representing the deviation. However, thirdly, if it greatly deviates from the specified central frequency only to be out of the transmission characteristic curve of the Fabry-Perot filter, the detected value is 0V. Thus, the detected voltage is 0V not only when the wavelength of the light signal precisely agrees with the specified central frequency, but when it be out of the transmission characteristic curve of the Fabry-Perot filter greatly deviating from the stabilized range. Therefore, it is impossible to correctly determine the stabilization of the wavelength only with the error signal. Namely, if the wavelength of a channel greatly deviates from the stabilized range, it cannot be detected at the transmission side, but at the receiving side only after receiving the signal. This delays the correction of such an error.
The following patents each discloses features in common with the present invention but do not teach or suggest monitoring the stability of the wavelength of a light signal in an optical communication system employing wavelength division multiplexing and having a wavelength stabilizing circuit as in the present invention: U.S. Pat. No. 5,080,505 to Epworth, issued on Jan. 14, 1992, entitled OPTICAL TRANSMISSION SYSTEM; U.S. Pat. No. 5,745,270 to Koch, issued on Apr. 28, 1998, entitled METHOD AND APPARATUS FOR MONITORING AND CORRECTING INDIVIDUAL WAVELENGTH CHANNEL PARAMETERS IN A MULTI-CHANNEL WAVELENGTH DIVISION MULTIPLEXER SYSTEM, U.S. Pat. No. 5,428,700 to Hall, issued on Jun. 27, 1995, entitled LASER STABILIZATION; U.S. Pat. No. 5,510,922 to Naito, issued on Apr. 23, 1996, entitled OPTICAL FREQUENCY STABILIZER AND OPTICAL FREQUENCY SELECTOR; U.S. Pat. No. 5,915,052 TO Ball, issued on Jun. 22, 1999, entitled LOOP STATUS MONITOR FOR DETERMINING THE AMPLITUDE OF THE SIGNAL COMPONENTS OF A MULTI-WAVELENGTH OPTICAL BEAM, U.S. Pat. No. 5,825,792 to Villeneuve et al., issued on Oct. 20, 1998, entitled WAVELENGTH MONITORING AND CONTROL ASSEMBLY FOR WDM OPTICAL TRANSMISSION SYSTEMS; U.S. Pat. No. 5,796,479 to Derickson et al., ISSUED ON Aug. 18, 1998, entitled SIGNAL MONITORING APPARATUS FOR WAVELENGTH DIVISION MULTIPLEXED OPTICAL TELECOMMUNICATION NETWORKS; U.S. Pat. No. 5,617,234 to Koga et al., issued on Apr. 1, 1997, entitled MULTIWAVELENGTH SIMULTANEOUS MONITORING CIRCUIT EMPLOYING ARRAYED-WAVEGUIDE GRATING; and U.S. Pat. No. 5,793,910 to Derr, issued on Aug. 11, 1998, entitled METHOD AND CIRCUIT ARRANGEMENT FOR FREQUENCY STABILIZATION OF A WDM MULTIPLEXER/DEMULTIPLEXER.
It is an object of the present invention to provide a circuit for monitoring the stability of the wavelength of a light signal in an optical communications system having a wavelength stabilizing circuit.
According to an aspect of the present invention, a circuit for monitoring the stability of the wavelengths of light signals in an optical communications system employing WDM and having a wavelength stabilization circuit, comprises: a plurality of channel transmitters each having a laser diode for generating a light signal at a particular frequency under the control of a laser diode driver; a WDM device for multiplexing the wavelengths by wavelength division; a Fabry-Perot filter having resonant frequencies continuously arranged with given free spectral ranges according to the central frequencies specified for the channels to resonate an output of the WDM device; a photodetector for converting the output light signals of the Fabry-Perot filter into corresponding electrical signals; and a quadrature phase shifter for shifting the phase of the output signal of the photodetector by 90xc2x0. Each of the channel transmitters further comprises: an oscillator for generating a dither signal of a frequency specified for the particular channel; a first PSD for effecting a phase sensitive detection of the dither signal and the output signal of the photodetector to generate an error signal corresponding to the first derivative signal of an Airy function; a first LPF for filtering the error signal to generate a first detected direct current voltage having a level representing the deviation of the output signal of the photodetector from the specified central frequency; an adder for adding the dither signal and first detected voltage to generate a bias signal supplied to the laser diode driver; a second PSD for effecting a phase sensitive detection of the dither signal and the output of the quadrature phase shifter to generate an intensity signal representing the intensity of the light signal transmitted by the WDM device; a second LPF for filtering the intensity signal to generate a second detected direct current voltage, and the monitor circuit monitoring the first and second detected voltages so as to indicate whether or not the wavelength of the light signal is stabilized based on the first and second detected voltages.
The present invention will now be described more specifically with reference to the drawings attached only by of example.